Polyarylene sulfide (also called “PAS” hereinafter), represented by polyphenylene sulfide (also called “PPS” hereinafter), is an engineering plastic having excellent heat resistance, chemical resistance, flame retardancy, mechanical strength, electrical characteristics, dimensional stability, and the like. PAS has been widely used in a wide variety of fields, such as electric/electronic devices and devices for automobiles, since PAS can be formed into various molded products, films, sheets, fibers, and the like by ordinary melt processing methods, such as extrusion molding, injection molding, and compression molding.
A known representative production method for PAS is a method of obtaining PAS such as PPS by performing a polymerization reaction on a sulfur source and a dihalo aromatic compound such as p-dichlorobenzene (also called “PDCB” hereafter) while heating (for example, temperature conditions of approximately 175 to 350° C.) using an aqueous mixture containing a polar organic solvent such as N-methyl-2-pyrrolidone (also called “NMP” hereafter) (Patent Documents 1 and 2). Other known methods for producing PAS with a high molecular weight include a two-stage polymerization method of performing a polymerization reaction while changing the polymerization temperature and the water content present in the polymerization reaction system, and a method of performing polymerization using a phase separation agent as necessary (Patent Documents 3 and 4).
PAS polymerization is often performed by loading an aqueous mixture containing a sulfur source and a dihalo aromatic compound into a polymerization device in the form of a roughly cylindrical reaction vessel (also called a “polymerization vessel”, a “polymerization can”, a “reaction can”, or the like), and it is necessary to continue to precisely control the composition of the polar organic solvent, raw material monomers, the produced PAS polymer, and the like inside the polymerization device, specifically, inside the reaction vessel, until polymerization is complete. Therefore, PAS polymerization is performed by supplying each of raw material monomers, a solvent (polar organic solvent, water, or the like), a polymerization aid, and the like from the opening end of one or a plurality of supply tubes provided at the upper part (also called a “lid” or the like) of the reaction vessel, adjusting the inside of the reaction vessel to a prescribed temperature and pressure and changing the temperature and pressure as necessary, additionally supplying raw material monomers, a solvent, or the like, and performing a polymerization reaction while sufficiently stirring. The produced PAS polymer is extracted from a discharge tube ordinarily provided at the base of the reaction vessel, and a PAS polymer product is obtained via washing, purification, or the like.
Chromium steel such as stainless steel, nickel, or the like has conventionally been used as a reaction vessel used for a polymerization reaction. However, in PAS polymerization, the reaction vessel, specifically, the inside wall of the reaction vessel, is in contact with corrosive materials such as a sulfur source or an alkali metal hydroxide, salt (NaCl), or the like in a high-temperature environment exceeding 200° C. for a long period of time. Thus, the material used to form the reaction vessel is required to have excellent corrosion resistance, chemical resistance, heat resistance, or stainless properties. Therefore, as disclosed in Patent Documents 4 to 6 and the like, in the production of PAS, a reaction device in which at least the part that comes into contact with the polymerization reaction solution is made of a titanium material or the like is considered preferable.
A reaction tube that is provided in the reaction vessel and opens into the reaction vessel is used to accurately load raw material monomers or other materials (for example, a strong alkali such as an alkali metal hydroxide) into the reaction vessel in preset prescribed amounts. Therefore, when the raw material monomers or other materials remain inside the supply tube (for example, near the tip opening) or adhere to the inside wall surface of the reaction vessel, there is a risk that the composition of the raw material monomers or the like used in the polymerization reaction may deviate from the prescribed composition. In particular, a supply tube for supplying a material (for example, various aids) in small amounts to the reaction vessel is required to have a function enabling the accurate supply of prescribed amounts.
After prescribed amounts of raw material monomers or other materials are loaded from a supply tube, a polymerization reaction or the like is performed inside the reaction vessel (as described below, a dehydration step or the like may also be performed). After the loading of an alkali metal hydroxide or the like into the reaction vessel from the opening end of the supply tube is complete, residual alkali metal hydroxide or the like may adhere to the opening end of the supply tube or the reaction vessel connected to the opening end, specifically, the inside wall surface or the like of the upper part (lid). In addition, when a polymerization reaction or the like is repeated multiple times inside the reaction vessel, the residual alkali metal hydroxide or the like adhering to the open end of the supply tube or the inside wall surface of the reaction vessel may accumulate.
It has been found that corrosion or the like may occur at the open end of the supply tube or the inside wall surface of the reaction vessel due to continuous contact with an accumulated alkali metal hydroxide or the like over a long period of time in the environment inside the reaction vessel. That is, as a result of continuous contact with a high-concentration strong alkali in a high-temperature and pressure environment exceeding 200° C., and as a result of the contact or accumulation of NaCl as a by-product and contact with hydrogen sulfide in the PAS production device, corrosion or the like may occur even with a material such as titanium which has excellent chemical resistance or corrosion resistance (an example of which is zirconium or the like, which is known as a corrosion-resistant material belonging to titanium-group elements).
When corrosion occurs at the open end of the supply tube or on the inside wall surface of the reaction vessel, there is a risk that corrosion will also progress to portions above the open end of the supply tube or inward in the thickness direction from the inside wall surface of the reaction vessel, which may diminish the safety of the reaction vessel as a pressure vessel. That is, when the mechanical strength required of the reaction vessel is lost due to corrosion, there is a risk that this may lead to an accident such as the breakage or destruction of the container or the leakage of the contents thereof. When the reaction vessel is formed using a laminated material such as titanium-clad steel as a based material, the loss of the titanium layer of the surface due to corrosion may cause the corrosion of the base material to progress with increasing speed. Further, there is a risk that a strong alkali, NaCl, or corroded strips accumulated on corroded parts such as the inside wall surface of the reaction vessel or the supply tube may drop into the reaction vessel, and in this case, there is a substantial risk of causing a decrease in the quality of the produced polymer such as PAS. Therefore, before the corrosion of the open end of the supply tube or the inside wall surface of the reaction vessel progresses, it is necessary to stop the operation of the reaction device and clean the open end and inside of the supply tube or the inside wall of the reaction vessel or to remove accumulated matter. In the event that the corrosion of the inside of the supply tube or the wall surface of the reaction vessel has progressed, it is necessary to stop the operation of the reaction device for a long period of time and replace the supply tube provided in the reaction vessel or to repair the reaction vessel. These lead to substantial decreases in production efficiency, so there is a demand for a solution.
Specifically, there is a demand to provide a PAS production device provided with a supply tube for loading corrosive materials such as a strong alkali, a strong acid, or hydrogen sulfide (also called “corrosive materials such as a strong alkali” hereafter) into a reaction vessel, wherein prescribed amounts of raw material monomers or other materials can be accurately loaded into the reaction vessel without causing decreases in production efficiency due to the replacement of the supply tube or the repair of the reaction vessel in response to the corrosion of the supply tube or the like.